Most vehicles are now equipped with a heating, ventilation and air-conditioning (HVAC) system for maintaining the air within the passenger compartment at a set desired temperature.
A problem that can occur in such vehicles equipped with an HVAC system, is that when the vehicle is left at stand-still after having used the HVAC refrigeration unit (i.e. operated the HVAC in the air-conditioning (A/C) mode), and then re-used after a certain period of time, large quantities of moisture laden air may be blown onto the windscreen if the HVAC system is set to windscreen defrost. This moisture laden air may condense on the glass area inside the car, thereby causing sudden fogging (often referred to as flash fogging) and thus reducing the driver's visibility.
This fogging effect is mainly caused by the evaporator of the HVAC refrigeration unit having become chilled during previous operation in the A/C mode, and therefore having a considerable amount of water condensed on the evaporator surface. During a vehicle stand-still or deactivation of the HVAC refrigeration unit, the evaporator warms up above ambient air temperature and the large quantities of water on the surfaces saturate the warmer air in the HVAC system around the evaporator. If the blower of the HVAC system is started in this condition, the moisture laden air is carried away from the evaporator and if air comes out from the defrost vent, then the moisture is deposited on the glass area.
In order to minimise fogging problems in vehicles equipped with an automatic climate control (ACC) unit for controlling the HVAC system, the ACC unit is generally programmed to blow air through the floor vents at vehicle start-up. Hence, the very moist air is not directed to the windscreen. It has however been observed that a certain amount of moisture laden air flows through the defrost vent—which cannot be tightly closed—so that windscreen fogging often still occurs. Furthermore, to rapidly reduce the moisture in the blown air, the ACC is generally programmed to operate the refrigeration unit (i.e. activating the compressor) from engine start-up; and to avoid re-fogging due to stopping of the refrigeration unit, the compressor is kept permanently active. Unfortunately, the permanent use of the refrigeration unit has a significant impact on fuel consumption, and is thus disadvantageous from both economical and environmental points of view. It is further to be noted that, at outside temperatures that will cause window fogging (typically between 3 and 15° C.), the air-conditioning, if it was not for preventing fogging, would normally be off, since a heated air flow is normally delivered by the HVAC system.
In order to avoid flash fogging, it is known to provide the defrost vent with a flap that can tightly close the defrost vent. However, the use of such a flap complicates the design of the HVAC system, increases its dimensions and its manufacturing costs.
U.S. Pat. No. 5,983,649 also addresses the fogging problem at engine start-up, and proposes to disable the HVAC blower, while the compressor of the refrigeration unit is activated to cool the evaporator. As soon as the evaporator has reached a sufficiently low temperature for condensing the moisture of the air contained in the HVAC system—which could need 10 to 18 seconds—the blower is switched on. This method does indeed avoid blowing moist air into the passenger compartment. At the end of this specific start-up sequence, the HVAC system is operated according to the main comfort programs, which thus typically keep the compressor active but deliver heated air flows. Indeed, deactivating the compressor when turning the blower on—i.e. after 10 to 18 seconds from engine start-up—would also, in most cases, cause fogging of the window. Hence, if the method of U.S. Pat. No. 5,983,649 provides an interesting solution for preventing fogging at engine start-up, it does not address the problem of fogging that often occurs when switching off the A/C mode (i.e. deactivating the compressor).